Magnetic Ink Character Recognition (MICR) technology is well-known. MICR toners contain a magnetic pigment or a magnetic component in an amount sufficient to generate a magnetic signal strong enough to be readable via MICR. Generally, the toner is used to print all or a portion of a document, such as checks, bonds, security cards, etc. For example, most checks exhibit an identification code area, usually at the bottom of the check. The characters of this identification code are usually MICR encoded. The document may be printed with a combination of MICR-readable toner and non-MICR-readable toner, or with just MICR-readable toner. The document thus printed is then exposed to an appropriate source or field of magnetization, at which time the magnetic particles become aligned as they accept and retain a magnetic signal. The document can then be authenticated by passing it through a reader device, which detects or “reads” the magnetic signal of the MICR imprinted characters, in order to authenticate or validate the document.
MICR toners contain a magnetic material that provides the required magnetic properties. It is important that the magnetic material retains a sufficient charge so that the printed characters retain their readable characteristic and are easily detected by the detection device or reader. The magnetic charge retained by a magnetic material is known as “remanence.” The “coercive force” of a magnetic material refers to the magnetic field H, which must be applied to a magnetic material in a symmetrical, cyclicly magnetized fashion, to make the magnetic induction B vanish. The coercivity of a magnetic material is thus the coercive force of the material in a hysterisis loop, whose maximum induction approximates the saturation induction. The observed remanent magnetization and the observed coercivity of a magnetic material depend on the magnetic material having some anisotropy to provide a preferred orientation for the magnetic moment in the crystal. Four major anisotropy forces determine the particle coercive force: magnetocrystalline anisotropy, strain anisotropy, exchange anisotropy, and shape anisotropy. The two dominant anisotropies are: 1) shape anisotropy, wherein the preferred magnetic orientation is along the axis of the magnetic crystal, and 2) magnetocrystalline anisotropy, wherein the electron spin-orbit coupling aligns the magnetic moment with a preferred crystalline axis.
The magnetic material should exhibit sufficient remanence once exposed to a source of magnetization, in order to generate a MICR-readable signal and have the capability to retain the signal over time. Generally, an acceptable level of charge, as set by industry standards, is between 50 and 200 Signal Level Units, with 100 being the nominal value, which is defined from a standard developed by ANSI (the American National Standards Instituter. A lesser signal may not be detected by the MICR reading device, and a greater signal may also not give an accurate reading. Because the documents being read employ the MICR printed characters as a means of authenticating or validating the presented documents, it is important that the MICR characters or other indicia be accurately read, without skipping or misreading any characters. Therefore, for purposes of MICR toner, remanence of the magnetic material should be at least a minimum of 20 emu/g to enable sufficient magnetization of the toner for MICR without use of excessively high pigment loadings in the toner. High pigment loadings in the toner poses difficulties in the toner preparation process and may negatively impact toner performance, and therefore high pigment loadings are undesirable. A higher remanence value in the toner corresponds to a stronger readable signal from the toner image.
Remanence tends to increase as a function of particle size and the density of the magnetic pigment coating. Accordingly, when the magnetic particle size decreases, the magnetic particles tend to experience a corresponding reduction in remanence. Achieving sufficient signal strength thus becomes increasingly difficult as the magnetic particle size diminishes and the practical limits on percent content of magnetic particles in the toner composition are reached. A higher remanence value will require less total percent magnetic particles in the toner formula, improve suspension properties, and reduce the likelihood of settling as compared to a toner formula with higher percent magnetic particle content.
Magnetite (iron oxide, Fe2O3) is a common magnetic material used in MICR toners. Magnetite has a low magnetocrystalline anisotropy, K1, of −1.1×104 J/m3. An acicular crystal shaped magnetite, in which one crystal dimension is much larger than the other, has an aspect ratio of the major to minor size axis of the single crystal (Dmajor/Dminor) of 2:1 or larger, helps to augment the magnetic remanence and coercivity performance in toners. Acicular magnetite is typically 0.6×0.1 micron in size along the major and minor axis, respectively, and has a large shape anisotropy (6/1). Typical loading of iron oxide in toners is about 20 to 40 weight percent of the total toner weight. However, due to the larger sizes and aspect ratio of acicular crystal shaped magnetite particles, they are difficult to disperse and stabilize into toners, especially in emulsion/aggregation processes. Moreover, spherical or cubic magnetites are smaller in size (less than 200 nm in all dimensions), but have low shape anisotropy (Dmajor/Dminor) of about 1. Consequently, because of the low overall anisotropy, both low shape anisotropy and low magnetocrystalline anisotropy, spherical or cubic magnetite have lower magnetic remanence and coercivity, and loadings higher than 40 weight percent of the total toner weight are often needed to provide magnetic performance. Thus, while spherical and cubic magnetite have the desired smaller particle size of less than 200 nm in all dimensions, the much higher loading requirement also makes them very difficult to disperse and maintain a stable dispersion. Moreover, such high loadings of the inert, non-melting magnetic material interfere with other toner properties, such as adhesion to the substrate and scratch resistance. Consequently, this worsens the suitability of magnetites for MICR toners.